Sealed batteries are largely classified into cylindrical batteries, prismatic batteries and pouch-type batteries depending on the shapes and materials of housing cases in which an electrode assembly that is the power generation element is accommodated. In particular, cylindrical batteries have found wide use in applications such as electric tools, power-assisted bicycles and electric vehicles. Thus, cylindrical batteries are required to exhibit high safety even when used under severe conditions.
A sealing unit of a cylindrical battery includes safety measures such as an explosion-proof valve, a current interrupt device and a PTC (positive temperature coefficient) element. Patent Literatures 1 to 4 listed later are example documents which disclose such sealing units. A sealing unit disclosed in Patent Literature 4 will be described below with reference to FIG. 6.
The sealing unit illustrated in FIG. 6 is composed of a metal case 66 in the form of a dish and, fitted therein, a stack including a metal plate 65, a valve member 63, a PTC element 62 and a terminal cap 61. A central portion of the valve member 63 is welded to the metal plate 65, and an outer peripheral portion of the valve member is insulated from the metal plate 65 by an insulating gasket 64. The metal plate 65 and the metal case 66 have a vent hole 65a and a vent hole 66a, respectively. This configuration allows the valve member 63 to experience a rise in the pressure inside the battery due to the generation of gas within the battery, and consequently the valve member 63 pulls the weld to the outside of the battery. If the pressure inside the battery reaches a prescribed value, the weld ruptures and the current path between the valve member 63 and the metal plate 65 is interrupted. If the pressure inside the battery continues to rise further, the valve member 63 ruptures starting from a thin portion 63a formed in the valve member 63, releasing the gas within the battery to the outside. This is how the current interrupt device and the explosion-proof valve disposed in the sealing unit are actuated.
While in the above example the weld between the valve member and the metal plate works as a current interrupt section, the current interrupt section may be constituted by a thin portion disposed in the metal plate as described in Patent Literatures 2 and 3. The pressure at which the current interrupt device is actuated depends on the fracture strength of the weld or the thin portion.